Even though the display driver supports SPI, we have not found any displays that could use it - the pins are simply not available on the display connector. Also, SPI would be incredibly, frustratingly slow for such a large screen. If you need an SPI display, check out many of our other offerings, nearly all support SPI-like protocols!
We suggest using a protoshield (with stacking headers) or a proto-screwshield to access the unused pins (2, 3, and analog 4 and 5). You can connect various i2c sensors or analog sensors to analog 4&5. You can also connect an i2c port expander to get more I/O pins. If you desperately need more pins, and you're careful, you can use the 8 datapins while the TFT isn't being written to - they are high-z and unused as long as the WR and RD pins are high